1. Field of the Invention
The present invention relates to a shadow mask of a cathode ray tube (CRT), and more particularly, to a structure of a slot feature of a shadow mask that is capable of improving characteristics of purity margin and brightness by projecting electron beams of certain shape on a screen formed at a rear side of a panel.
2. Description of the Background Art
In general, the shadow mask is installed inside a Braun tube used for a TV or a monitor set and performs a color sorting to mount electron beams generated from an electron gun on a desired fluorescent material surface of the screen.
As shown in FIG. 1, the CRT includes a fluorescent face 4, to which a front glass called panel 1 and a rear glass called a funnel 2 are coupled, being emitted inside the panel 1; an electron gun 10, the source of the electron beams 6 for emitting the fluorescent face; a shadow mask 3 for sorting a color so as to emit a certain fluorescent face, and a frame 7 for supporting the shadow mask.
A spring 8 for rendering a frame assembly to be coupled to the panel 1 and an inner shield 9 for reducing an influence of an external terrestrial magnetism during the operation of the CRT are coupled to the frame, and the panel and the funnel are sealed with a high vacuum.
The operational principles of the CRT will now be described.
The electron beam 6 is landed on the fluorescent face 4 formed inside the panel 1 by an anode voltage applied to the CRT from the electron gun 10 inserted in a neck portion (with no reference numeral). At this time, before the electron beam 6 reaches the fluorescent face 4, it is deflected up, down, left and right by a deflection yoke 5 to display an image.
Pole magnet 11 corrects the proceeding trajectory so that the electron beam 6 can accurately hit the fluorescent face 4, thereby preventing deterioration of a color purity.
A reinforcing band 12 is coupled at an outer circumferential surface of a junction portion between the panel 1 and the funnel 2 to reinforce the junction.
As shown in FIG. 2, the fluorescent face 4 inside the panel 1, a graphite band 4a, and red, green and blue fluorescent material 4b are coated in a stripe form.
The shadow mask 3 has a dome shape, maintaining a certain space from the inside of the panel 1, including, as shown in FIG. 3, an effective surface portion 3b with a plurality of slots in a stripe form formed at the central portion, a mask skirt portion (not shown) almost vertically bent from the marginal portion 3c at the outermost portion of the marginal portion 3c. 
The shadow mask 3 typically has a thickness of about 0.1˜0.3 mm.
The plurality of slots 3a, holes through which the electron beam 6 passes, are formed with a certain arrangement on the effective surface portion 3b of the shadow mask 3.
With reference to FIG. 4, the red, green and blue electron beams 6 are focussed on the fluorescent material face 4 through the slots 3a formed at one side of the shadow mask 3.
Thus, when the electron beams 6 hit the fluorescent material face 4 of the panel 1 after passing the shadow mask 3, the electron beams 6 formed on the fluorescent material face 4 have a similar shape as the mask slot 3a. 
That is, the shape of the electron beam 6 before passing the slots 3a of the shadow mask 3 is similar to a circle in view of its section, and the section of the shape of the electron beam 6 focussed on the fluorescent material face 4 is formed according to the shape of the slot 3a (refer to FIGS. 6B and 6D).
As shown in FIG. 5, in the conventional CRT, the angle (θ1) at which the deflected electron beam 6 is made incident on the shadow mask 3 is close to a right angle, while an angle (θ2) at which the deflected beam is made incident on the mask becomes small in a flat type CRT.
Accordingly, the shape of the electron beam 6 formed on the panel 1 is different from the shape of the slot 3a formed at one side of the shadow mask 3 depending on the deflected angle, the distance between the shdow mask 3 and the inner side of the panel 1 and the distance between the deflection yoke 5 and the shadow mask 3, and the left and right shapes of the electron beam 6 are not identical to each other.
As shown FIGS. 6A through 6D, such a phenomenon does not take place on the entire screen, and generally, the longer it is distanced from the center, the more serious the phenomenon is.
FIGS. 6A and 6C show a shape of the slot 3a uniformly formed at the central portion and the marginal portion of the shadow mask 3. FIG. 6B shows a shape of the electron beam 6 focussed at the central portion of the fluorescent face 4 according to the slot feature of 6A, and FIG. 6D shows a shape of the electron beam 6 focussed at the marginal portion of the fluorescent material face 4 according to the slot feature of 6C
As for the color CRT, the outer surface of the panel is advanced to a flat surface from the past curved surface in order to prevent degradation and visual fatigue, and as the color CRT is adapted for various uses, there is a tendency for advancing to a high quality with a fine pitch that can adopt frequencies of various modes.
Accordingly, the tendency is for the inner curvature of the panel 1 to become flat compared to a general CRT, and the curvature of the shadow mask 3 also becomes flat.
As the curvature becomes flat, the incident angle of the electron beam 6 is gradually changed to an acute angle as it goes toward the marginal portion of the screen. Accordingly, after the electron beam 6 passes the slot 3a of the shadow mask 3, when the electron beam 6 is projected on the inside of the panel 1, the shape of the electron beam 6 is distorted.
In addition, in a fabrication process such as a series of operations such as a deflection yoke engagement and a landing correction, due to the distortion in the shape of the electron beam 6, there occurs a difference between a landing level determined by operators' naked eyes and an actual landing value, resulting in that a process time is lengthened due to the increase in the corresponding working time and an operation level is also degraded (refer to FIG. 7).
In FIG. 7, reference numeral 6a shows a shape of the electron beam 6 which has passed the shadow mask 3, and reference numeral 20 shows a shape after a portion of the electron beam 6 is absorbed into the graphite band 4a constituting the fluorescent material face 4.
In this case, since the shape of the left and right transmitted electron beam 6a is shown that the outer side has a circular arc for the central portion, operators may misjudge its actual landing value during a fabrication process.
In order to solve the problems, Japanese patent publication No. 2-86027 solves the problem in such a manner that a cut-out portion is additionally formed toward the marginal portion of the shadow mask with respect to the feature of each slot. But a problem arises in that the electron beam irradiated on the screen after passing the central portion of the shadow mask fails to have a perfectly straight line.